Let's say you're a Volga German, a Russdeutscher, one of those ethnic Germans who descends from German settlers invited into Russia by Catherine the Great to serve as a buffer between the Russians and the Chechens. Things happen, a couple of world wars, the collapse of Communism, and in the newly independent republics, all the jobs are for the natives, not for the transplants, i.e., the Volga Germans. So, you decide for the sake of your children to move back to Germany, a land neither you nor your parents have ever seen. What happens to you?
Well, since you're now on the dole, the German government gets to decide where you live. They break up your extended family, which has just spent a generation living on one street, and settle each nuclear family in a different city or suburb. Why? To force you to interact with Germany's Germans, to re-Germanize you. It's basically a planned program of intensive assimilation. And it is very effective. Within five years, you give up many of your old ways of dress and speech. Your children are all either in school or have good jobs. Within ten years, all that remains to distinguish you outwardly from the other Germans is your willingness and aptitude for hard work and thrift (fixing up your new house yourself), and a slight accent. And well, maybe you're still more likely to vote CDU than SPD, but not much more likely.
But if you are a Muslim immigrant from the Gulf states, you might wind up on the dole, but there is no intensive program of assimilation for you. You are left in a virtual ghetto, you keep your old speech, and you keep your old values. Except, maybe as a second-class citizen of Germany (even though you may be a third generation citizen), you may become radicalized.
Now maybe intensive assimilation would be poorly received by both the Muslim immigrants and by the native Germans. I know one ethnic German who was assaulted by other Germans in a German city, because he looked "like one of those Turks." Perhaps the new Muslim immigrants would not fare well in the isolation that would be part of an intensive assimilation program. And perhaps the Muslim community would view intensive assimilation as an attempt at cultural (and religious) genocide on the part of the Germans. Or maybe it's just that the German people and government don't want the Muslim immigrants to be assimilated, or just don't care, or maybe they just don't think about it.
Without intensive assimilation of non-ethnic German immigrants, Germany is allowing the creation of a reservoir for radicalism to take hold. Remember that Mohammed Atta, the ringleader of the 9/11 attacks, was trained in engineering in Germany. He was not a radical when he entered Germany, but he was certainly radicalized by the time he left. If Muslims were more intensively assimilated, they would remain Muslim, but they would perhaps absorb some of the West's humanism, rather than merely be repelled by its secularism and consumerism. Such might have been the case with Atta.
Now, I could as well pick on any European country regarding the failure to assimilate new immigrants. I only single out Germany, because I happen to be more familiar with it. For that matter, the United States is no longer very intentional about assimilation, about "The Great Melting Pot." But the American culture is somehow better at assimilating immigrants anyway, mostly because everyone in America is decended from immigrants, unless their ancestors walked across the land bridge between Asia and Alaska at the end of the last Ice Age.